How to find the largest number in an array?

Question

I wrote a code that takes in a list of numbers from the user and console logs the largest number, but it is not showing the correct output. Let me explain this code. This is a code that asks user for numbers (input) until they write quit. The numbers given by the user is stored in an array called numberList. Then I wrote a code that finds out the largest number in that array. But for some reason it doesn't work. Please help me out..

function add() {
  var addNum = prompt(`What number would you like to add?`);
  numberList.push(addNum);
}

function display() {
  console.log(numberList);

}

var numberList = [];

var start = prompt(`Would you like to start the programme? y/n`);
var userInput;

if (start === 'y') {
  while (userInput !== 'quit') {
    userInput = prompt(`Action: add, display, quit`);
    if (userInput === 'add') {
      add();
    } else if (userInput === 'display') {
      display()
    } else if (userInput === 'quit') {
      userInput = 'quit';
      alert(`Thank you for using the programme`)
    } else {
      alert(`Please enter a valid input`);
    }
  }
} else if (start === 'n') {
  alert('As you say');
} else {
  alert(`Please enter a valid input`);
}

var largestNum = 0;

for (i = 0; i < numberList.length; i++) {
  if (numberList[i] > largestNum) {
    largestNum = numberList[i];
  }
}
console.log(`The largest number is ${largestNum}`);